Temperature- and Light-Induced Spin Crossover Observed by X-ray

Letter pubs.acs.org/JPCL

Temperature- and Light-Induced Spin Crossover Observed by X‑ray Spectroscopy on Isolated Fe(II) Complexes on Gold Ben Warner,⊥,† Jenny C. Oberg,⊥,† Tobias G. Gill,⊥,‡ Fadi El Hallak,⊥ Cyrus F. Hirjibehedin,⊥,†,‡ Michele Serri,# Sandrine Heutz,# Marie-Anne Arrio,¶ Philippe Sainctavit,¶ Matteo Mannini,§ Giordano Poneti,§,∇ Roberta Sessoli,§ and Patrick Rosa*,∥ ⊥

London Centre for Nanotechnology, †Department of Physics & Astronomy, and ‡Department of Chemistry, University College London, London, United Kingdom # Department of Materials and London Centre for Nanotechnology, Imperial College London, London, United Kingdom ¶ Institut de Minéralogie et de Physique des Milieux Condensés, UMR7590, Université Pierre et Marie Curie, Paris, France § La. M. M., Dipartimento di Chimica “U. Schiff ” & INSTM RU Firenze, University of Florence, Sesto Fiorentino, Italy ∇ Department of Applied Sciences and Technology, Guglielmo Marconi University, Rome, Italy ∥ CNRS, Univ. Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France S Supporting Information *

ABSTRACT: Using X-ray absorption techniques, we show that temperature- and light-induced spin crossover properties are conserved for a submonolayer of the [Fe(H2B(pz)2)2(2,2′-bipy)] complex evaporated onto a Au(111) surface. For a significant fraction of the molecules, we see changes in the absorption at the L2,3 edges that are consistent with those observed in bulk and thick film references. Assignment of these changes to spin crossover is further supported by multiplet calculations to simulate the X-ray absorption spectra. As others have observed in experiments on monolayer coverages, we find that many molecules in our submonolayer system remain pinned in one of the two spin states. Our results clearly demonstrate that temperature- and light-induced spin crossover is possible for isolated molecules on surfaces but that interactions with the surface may play a key role in determining when this can occur.

SECTION: Physical Processes in Nanomaterials and Nanostructures

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insulator16 such as Cu2N,26 exhibit voltage-induced switching. However, electrically induced switching of SCO molecules placed directly on metal surfaces was not possible,12,16 though the coexistence of molecules pinned in the HS and LS states has been observed using low-temperature X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD).16 In addition, while temperature-driven SCO has been detected for a different FeII complex evaporated on HOPG (highly ordered pyrolytic graphite),27 evidence of lightinduced spin state switching for surface-confined SCO molecules is still lacking. Here, we report the detection of temperature- and lightinduced SCO for isolated molecules of complex [Fe(H2B(pz)2)2(bipy)] 1 (Scheme 1) on Au(111), using X-ray absorption techniques along with multiplet calculations. Such results, which have not yet been reported, offer direct evidence that temperature- and light-induced SCO, in addition to electrically induced switching,16 is indeed possible for isolated

pin crossover (SCO) complexes are promising building blocks for spintronic1 and high-density memory devices because they contain a d4 to d7 transition-metal ion that can be reversibly switched between two distinct spin states, low-spin (LS) and high-spin (HS).2,3 This phenomenon can be driven using a variety of external inputs, including temperature,4,5 light,6−8 pressure,9 magnetic field,10 mechanical stress,11 and charge flow.12−17 Recently, there has been a surge of interest in the study of the conversion properties of nanostructured SCO materials, in the form of nanoparticles,18−22 thick films,23−25 and even isolated molecules either bridging nanogaps13−15 or on surfaces.16,17 Integration of switchable molecular materials in nanoscale devices requires the retention of their conversion properties once deposited on a solid substrate. SCO systems are known to display conversion features hugely dependent on the environment and can be easily perturbed by the molecule’s interaction with the surface. Recently, clean ultrahigh vacuum (UHV) deposition of thermally evaporable SCO molecules has allowed preparation of submonolayer deposits. By using scanning tunneling microscopy (STM), it has been shown that SCO molecules, decoupled from underlying metallic surfaces, either by another layer of molecules12 or by a thin © 2013 American Chemical Society

Received: March 12, 2013 Accepted: April 17, 2013 Published: April 17, 2013 1546

dx.doi.org/10.1021/jz4005619 | J. Phys. Chem. Lett. 2013, 4, 1546−1552

The Journal of Physical Chemistry Letters

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Scheme 1

molecules even on metallic substrates. In the bulk, SCO behavior is quite sensitive to the molecules’ environment, and our results suggest that for submonolayers, both interactions with the surface and neighboring molecules may play a role in inhibiting the SCO phenomenon. In order to detect thermally and light-driven SCO, XAS was employed, as done previously on the model SCO complex [Fe(NCS)2(phen)2] in the bulk phase,28−30 due to the technique’s element, oxidation, and spin state selectiveness. Most importantly, the extreme sensitivity afforded by the total electron yield (TEY) detection of X-ray absorption31−35 allows for the investigation of submonolayer deposits, as recently reported for FeII-based SCO complexes sublimed on Cu16 and HOPG.27 Using variable-temperature X-ray absorption spectra, we examined a submonolayer coverage evaporated in situ under UHV conditions on Au(111) before and after irradiation with visible laser light. We compare these results to those obtained from two other samples, (1) a single crystal finely scratched on gold foil, which we use as a spectroscopic bulk reference, and (2) a 300 nm thick film sublimed ex situ on copper foil to check the preservation of the structure and properties of the complex. Experimental spectra were then compared to the ones obtained using multiplet calculations.36−38 The variation of the L2,3 edge spectra for the bulk sample over the range of the thermal spin crossover (100−300 K) is reported in Figure 1a (see also Figure S1 in the Supporting Information). Both L2 (721.5−721.9 eV) and L3 maxima (708.7−710.0 eV) shift to higher energies when the temperature is decreased, with an L2 edge less structured and more intense at 150 K. The branching ratio,39,40 defined as I3/(I3 + I2), where I stands for the integrated intensity at the corresponding L edge, decreases from 0.78 at 300 K to 0.64 at 100 K. As observed for complex [Fe(NCS)2(phen)2]28−30 and recently for a monolayer of another complex [Fe(NCS)2L],28 all of these features are associated with the reversible transition from a t2g4eg2 configuration (S = 2) at 300 K to t2g6eg0 (S = 0) at low temperature. These results are in good agreement with previous X-ray photoemission data.41 Spectra measured at 300 K before and after the temperature cycle are identical, confirming that the SCO process is fully reversible with no X-ray-induced degradation. Experimental spectra at 300 and 100 K show in both cases excellent agreement with spectra resulting from the ligand field multiplet calculations,42 with an Oh crystal field parameter 10Dq of 1 eV for the HS configuration and 2.2 eV for the LS one. These are similar to values reported for complex [Fe(NCS)2(phen)2], respectively 0.5 and 2.2 eV36 or 0.9 and 2.2

Figure 1. Normalized spectra of 1 (absorption maximum) at 300 (red line), 100 (blue line),and 10 K (green line) after illumination (658 nm cw laser, 15 mW·cm−2) for (a) the bulk and (b) the 300 nm thick film with (c) the corresponding calculated spectra. Spectra are shifted vertically for clarity.

eV.30 These 10Dq values can be compared with the range predicted for the occurrence of SCO from ligand field theoretical considerations, 10DqHS ≈ 1.36−1.55 eV and 10DqLS ≈ 2.36−2.73 eV.3 The discrepancy for 10DqHS has been explained by the fact that XAS is only sensitive to the excited state crystal field splitting for high-spin complexes.36 Spectra measured on the thick film prepared ex situ are similar to those in the bulk and show comparable temperature dependence (Figure 1b and Figure S1, Supporting Information). Nevertheless, the thick film spectrum is slightly different from that of the bulk compound; shoulders on the high-energy side (at 300 K) or at the low-energy side (at 100 K) of the L3 absorption peak are likely associated with a small fraction of decomposition product, which may be caused by air exposure of this sample prepared ex situ. The analysis of the temperaturedependent spectra as weighted sums of the bulk spectra at 300 1547



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nm),46−48 a thickness much smaller than the light penetration depth. The relaxation behavior of the film and the bulk is similar to what is observed with reflectometry or photomagnetic measurements, with similar TLIESST temperatures.49 The evaluation of the magnetic response through XMCD spectra fully supports our assignment of HS and LS states. Compound 1 behaves as a paramagnet; the XMCD signal measured is weak at 300 K, increases until SCO occurs, and then becomes very weak down to 10 K. It increases again with photoexcitation at low temperatures, with spectra corresponding quite well with the calculated spectrum, and then decreases with temperature-induced relaxation (Figures S7−S9, Supporting Information).50 With the experimental method fully established on the bulk and the evaporated thick film, we moved to a submonolayer coverage sample grown on Au(111), which we prepared in situ at the ID08 ESRF beamline. XAS spectra allowed us to evaluate the amount of molecules sublimed onto the Au surface by measuring the Fe edge jump (i.e., the ratio of intensities before and after the L3 edge),51,52 provided that absorption for ions other than the Fe atoms of the molecule is negligible compared to the Au(111) substrate absorption, which is a valid assumption here. We found an edge jump of 2.3%, which is lower than the ones reported for another Fe(III)-based molecular system prepared by monolayer self-assembly35 and that for 0.8 monolayers of another SCO complex on HOPG.27 As detailed in Figure S11 in the Supporting Information, this edge jump, being 100 times smaller than the ones found for the bulk and thick film samples, gives an approximate coverage about 0.03−0.14 monolayers, thus allowing one to safely exclude the presence of multilayer islands that would moreover be clearly visible on STM images.53 Figure 3 reports for this sample the evolution of XAS spectra between 100 and 300 K. The analysis of the spectra in Figure 3a (relative ratio of peaks at 708.7 and 710.0 eV) points out that even at 300 K, we have a mixture of molecules in both spin states, as also reported previously for a related SCO complex.12 When lowering the temperature from 300 to 100 K, the change of this ratio and the increase of the intensity of the L2 peak show clearly that the HS/LS ratio decreases, according to the expected SCO behavior, although these changes are not as large as they are in the bulk or thick film samples. This clear nonmonotonic temperature dependence of XAS excludes spurious temperature-induced changes such as sample damage under the beam or ice deposition on the sample. Thus, thermal spin crossover is clearly observed but only for a fraction of the molecules covering the surface. The remainder is trapped in either the LS or the HS state, showing that for some molecules, the electronic state and/or the geometry are affected by the surface. As detailed in Figure 3b and c, spectra are excellently reproduced by a linear combination of the HS and LS spectra of the bulk compound, showing that we obtained intact molecules of compound 1. Figure 4 reports for this sample the evolution of XAS spectra at 10 K after irradiation with visible light, compared to the spectrum at 100 K. The spectrum at 10 K (see Table S13, Supporting Information) before irradiation as compared to the earlier spectrum at 100 K shows an increase of the HS fraction, caused by the SOXIESST effect already seen for the bulk. When irradiating with visible light (cw laser, 658 nm, 15 min), the effect is also in this case larger than the SOXIESST effect, bringing the LS/HS ratio back to values observed at room temperature. The behavior of the changes in the spectra with

and 100 K, chosen as representative of the HS and LS states, respectively, allows for the extraction of the temperature dependence of the HS fraction (Table S2, Supporting Information). For the thick film, the shoulder signals were found to be temperature-independent and thus do not affect the switching behavior. We extracted this spurious contribution (Figures S3 and S4, Supporting Information) and subtracted it from all spectra before evaluating the HS fraction (Table S5, Supporting Information). Figure 2 summarizes the result of these analyses, together with the HS fraction derived from magnetometric investigations.43 The agreement between experimental data obtained with different techniques is excellent. Spectra of the bulk and the thick film at 10 K after laser irradiation, with the corresponding spectrum resulting from multiplet calculations, are also shown in Figure 1. Before irradiation, in the dark, the bulk is almost completely in the LS state, though for the thick film, a more significant HS component is detected (Figure 2). This is the result of some

Figure 2. HS fraction versus temperature derived from the linear interpolation of the XAS spectra for the bulk compound before (filled black squares) and after illumination (empty black squares) and the 300 nm thick film before (filled blue triangles) and after illumination (empty blue triangles) compared to the magnetometric results on the bulk compound before (full line) and after illumination (red dotted line).

spin conversion resulting from X-ray irradiation, the soft X-ray induced excited spin state trapping (SOXIESST) already reported on [Fe(NCS)2(phen)2]29 as well as for different switchable molecular species.44 The effect is already visible at 50 K; however, we do not observe the irreversible X-rayinduced photoconversion that was reported for [Fe(NCS)2(phen)2].29 As already seen by photomagnetic measurements on a thick film of compound 1 on Kapton,25 the film at 10 K is more sensitive to photons, here at X-ray wavelengths, than the bulk. The cw laser irradiation at 10 K (658 nm, 15 mW·cm−2) readily populates the HS state by the LIESST effect (lightinduced excited spin state trapping).7,8,45 After switching off the laser irradiation, the relaxation of the HS trapped state could be followed with increasing temperature (see Figure S6, Supporting Information), yielding the fraction of the metastable HS state reported in Figure 2. The efficiency of the light-induced conversion is close to 100%, as compared to some 60−80% as determined by magnetic measurements. This is not surprising because, similarly to what is observed in reflectometry measurements,25 TEY is probing the topmost layers of the sample (typically 1−5 1548



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Figure 4. (a) Comparison of spectra at the Fe L3 edge (705−715 eV) for the submonolayer sample of 1 at 100 and then at 10 K after laser irradiation (658 nm cw laser, 15 mW·cm−2) (blue and green lines, respectively). (b) Experimental spectra at 10 K after irradiation (black line), with corresponding HS (red line) and LS (blue line) components and the linear combination for a HS/LS ratio of 69:31 (dashed gray line). Spectra were normalized at 705 eV (see Table S13 in Supporting Information for more details).

Figure 3. (a) Comparison of spectra at the L3 edge (705−715 eV) for the submonolayer sample of 1 at 300 and 100 K (red and blue lines, respectively); (b) T = 300 K experimental spectrum (black line), with corresponding HS (red line) and LS (blue line) components and the linear combination (dashed gray line) for a HS/LS ratio of 69:31 at 300 K; (c) T = 100 K experimental spectrum (black line) with corresponding HS (red line) and LS (blue line) components and the linear combination (dashed gray line) for a HS/LS ratio of 56:44 at 100 K. Spectra were normalized at 705 eV (see Table S13 in Supporting Information for more details).

Figure 5. HS fraction versus temperature derived from the linear interpolation of the XAS spectra for the submonolayer sample; filled squares are before laser irradiation, and green empty circles are after laser irradiation (numbers label the acquisition order). As a guide to the eye, the shifted and rescaled temperature dependence of the thermal- and photoinduced HS fractions of the bulk specimen are drawn as faded bands. Note that the HS fraction scale ranges from 50 to 80% for clarity.

light exposure and the reversal of these changes upon increasing the temperature after irradiation further support our assignment of this phenomenon to spin crossover. We applied the same procedure previously employed for the bulk sample and thick film to quantify this HS molar fraction in the submonolayer. Figure 5 and Table S13 (Supporting Information), reporting the evolution with temperature of the HS fraction of the submonolayer, evidence a transition profile in excellent agreement with the ones previously found. Error bars on individual points are smaller than the observed

variations with temperature and light irradiation, thus excluding any fit bias. From this analysis, we observe that 20% of the molecules show SCO behavior similar to that observed on the bulk or the 1549



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thick film, while the remaining molecules are trapped in a definite spin state, 53(1)% in HS and 27(2)% in LS. Although only a fraction of the deposited molecules exhibits SCO behavior, it has been possible to characterize in detail their conversion process. In particular, they have revealed the following. (i) thermal SCO with a transition temperature at around 160 K, similar to the one observed for the bulk, an indirect piece of evidence that we have intact molecules on the surface but with a seemingly smoother temperature dependence, as observed on highly diluted SCO systems;54 (ii) SOXIESST at low temperature in the dark causing a partial conversion to the HS state of the switchable molecules; (iii) a quick and full conversion of the remaining molecules to the HS state upon cw laser irradiation at 658 nm; (iv) complete relaxation of the metastable photoinduced state to the LS state between 40 and 65 K, comparable with the bulk phase where TLIESST = 52 K; this is expected if the molecule has preserved its structure on the surface because this relaxation behavior was shown to depend primarily on the first coordination sphere of the metal ion;55 (v) a reversible transformation, with two 300 K spectra measured before and after thermal- and light-induced SCO showing close HS/LS proportions. In summary, we have observed thermal- and light-induced SCO on submonolayer coverage of complex 1 on Au(111). The transition appears to be fully reversible, and its features are virtually identical to the ones observed on the bulk. This demonstrates that spin crossover complexes retain their properties at the single-molecule level even on metallic substrates, which allows for the possibility of addressing their spin states through temperature or light in spintronic applications,56 while electrical switching can also be envisaged. We observed however also a mixture of HS and LS pinned states throughout the full temperature range for our submonolayer coverage. This suggests a certain sensitivity of the switchable behavior of SCO to the environment. In fact, switching was not observed for a full monolayer of complex 1 on Au(111)12 and of complex [Fe(NCS)2(phen)2] on Cu(100)16 but was observed for a monolayer on HOPG of complex [Fe(NCS)2L].27 The latter complex shows a much more gradual SCO, hinting that cooperative interactions between the molecules may play a role in blocking SCO. Future theoretical and experimental studies, particularly with probes that can determine the precise binding configuration of the molecules on a given surface for various coverages, may shed more light on the reasons why some molecules remain trapped while others undergo spin crossover. The observation that the spin state can be switched by light also in the case of molecules deposited on a conducting substrate increases significantly the interest in SCO molecules as potential building blocks for single-molecule devices.

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spectra at 300 K for all samples (Figure S11), STM images of the submonolayer sample (Figure S12), and deconvolution results for the submonolayer (Table S13). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Address: ICMCB-CNRS, 87 av. Dr. A. Schweitzer, 33608 Pessac Cedex, France. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

Funding was from ANR-11-JS07-013-01-CHIROTS, Région Aquitaine (P.R.); EPSRC Grants EP/D063604/1, EP/ H002367/1 (B.W., J.C.O., T.G., F.E.H, C.F.H.), E.P/ H002022/1 (M.S, S.H), and EP/F04139X/1 (S.H.); ERC Advanced Grant MolNanoMaS Proj. N. 267746 (M.M, G.P., R.S); and NanoPlasMag Proj. RBFR10OAI0 (M.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge ESRF for providing beam time under Projects HE3523 and HE3754, and we thank Julio Cezar Criginski, Flora Yakhou, and Kurt Kummer for their help during beam time. We also thank Joris van Slageren for stimulating discussions.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Full experimental details, bulk and thin film XAS spectra variation with temperature (Figures S1, S3, S4), deconvolution results for the bulk (Table S2) and the thick film (Table S5), relaxation of the photoinduced HS state for the bulk monitored by XAS (Figure S6), XMCD spectra for the thick film (Figure S7) and the bulk (Figure S8), comparison of XAS/XMCD spectra with multiplet calculations (Figure S9), XAS/XMCD spectra of the spurious component (Figure S10), unscaled XAS 1550



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(47) Ono, S.; Kanaya, K. The Energy Dependence of Secondary Emission Based on the Range-Energy Retardation Power Formula. J. Phys. D.: Appl. Phys. 1979, 12, 619−632. (48) Frazer, B. H.; Gilbert, B.; Sonderegger, B. R.; De Stasio, G. The Probing Depth of Total Electron Yield in the Sub-keV Range: TEYXAS and X-PEEM. Surf. Sci. 2003, 537, 161−167. (49) TLIESST temperatures are seen qualitatively to be close to 50 K, similar to the 52 K observed for the bulk in photomagnetic measurements. The LIESST measurement is a kinetic experiment, thus very sensitive to the time scale of the measurement, which is above several minutes for an XAS setup, all the more in the thermally activated regime, expected above 40 K for compound 1. (50) The 100 K thick film XMCD spectrum, with no contribution from compound 1, shows a paramagnetic degradation product (Figure S10 in the Supporting Information). (51) The absorption stems from the sum rule on the number of holes created, see: Thole, B. T.; van der Laan, G.; Fabrizio, M. Magnetic Ground-State Properties and Spectral Distributions. I. X-rayAbsorption Spectra. Phys. Rev. B 1994, 50, 11466−11473. (52) Thole, B. T.; van der Laan, G. Magnetic Ground-State Properties and Spectral Distributions. II. Polarized Photoemission. Phys. Rev. B 1994, 50, 11474−11483. (53) STM images (see Figure S12, Supporting Information) show no evidence of aggregates, fragments of monolayers, or multilayer islands. Being unable to identify isolated molecules is not surprising in measurements at room temperature, where the molecules may not be immobilized the way that they are at cryogenic temperatures (see refs 16 and 25). Furthermore, the STM setup is directly connected to the synchrotron spectrometer, which makes it difficult to perform measurements with low vibrational noise. (54) Martin, J.-P.; Zarembovitch, J.; Dworkyn, A.; Haasnoot, J. G.; Codjovi, E. Solid-State Effects in Spin Transitions: Role of Iron(II) Dilution in the Magnetic and Calorimetric Properties of the Series [FexNi1−x(4,4′-bis(1,2,4-triazole))2(NCS)2]·H2O. Inorg. Chem. 1994, 33, 2617−2623. (55) Létard, J.-F.; Guionneau, P.; Nguyen, O.; Sánchez Costa, J.; Marcén, S.; Chastanet, G.; Marchivie, M.; Goux-Capes, L. A Guideline to the Design of Molecular-Based Materials with Long-Lived Photomagnetic Lifetimes. Chem.Eur. J. 2005, 11, 4582−4589. (56) Sanvito, S. Molecular Spintronics. Chem. Soc. Rev. 2011, 40, 3336−3355.

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Temperature- and Light-Induced Spin Crossover Observed by X-ray

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